1. Field of the Invention
The present invention relates to foamed polyolefin resin beads which are excellent in fusion properties between beads at the time of molding in a mold, capable of providing a molded foamed article which is excellent antistatic performance, has no deterioration of the antistatic performance with elapse of the time, whose antistatic performance is not humidity dependent, does not contaminate packaging products, has a good molded foamed article surface, and has excellent mechanical properties. The present invention provides foamed polyolefin resin beads and a molded article obtained by molding the beads in a mold which are used as shock absorbing materials, heat insulating materials, or packaging materials in the electronic or automotive industry or useful for application to returnable boxes.
2. Description of the Related Art
Foamed polyolefin resin beads and a molded foamed article produced by molding the foamed polyolefin resin beads in a mold have been widely used for packaging of electronic devices such as OA instruments and parts or precision instruments and parts and further used as cushioning packaging materials. Polyolefin resin is easily charged, which causes adhesion of dust and damage to equipment parts. Therefore, a molded foamed article composed of foamed polyolefin resin beads to which antistatic performance is given has been used.
Examples of the method for imparting the antistatic performance to the foamed polyolefin resin beads include a method for applying a surfactant to the surface of the molded article and a method for molding foamed beads in which a polyolefin resin into which the surfactant is kneaded is foamed. A molded article composed of foamed beads in which the polyolefin resin into which the surfactant is kneaded has been used as a suitable material with antistatic performance. However, when the antistatic performance is achieved by the surfactant, the surfactant present on the surface of the molded article surface adsorbs moisture in the air. Therefore, it is difficult to produce the antistatic effect under an environment of low humidity, particularly in winter. Additionally, there have been problems of contamination due to the adhesion of the surfactant to packaging products and decrease in performance with age.
In the method for applying the surfactant to the surface of the molded article, the surfactant is easily flaked off from the surface of the molded article and the antistatic effect cannot be produced after the separation.
As a method for solving the problems of the adhesion of the surfactant to packaging products, contamination of packaging products, and flaking of the surfactant when the surfactant are used, foamed polyolefin resin beads containing a hydrophilic polymer and the surfactant and the molded foamed article obtained by molding the foamed polyolefin resin beads in a mold are disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2000-290421. The invention described in JP-A No. 2000-290421 is that the surfactant is trapped between molecules of the hydrophilic polymer, thereby preventing the surfactant from transferring to packaging products.
Alternatively, foamed resin beads are produced by foaming and expanding resin particles to which polymer type antistatic agent containing the hydrophilic polymer as an antistatic agent is added, thereby preventing the surfactant from transferring to packaging products and antistatic performance from being reduced, which is well known.
A technique which gives antistatic performance by coating the surface of foamed polyolefin resin beads with an adhesive resin containing the polymer type antistatic agent is disclosed in JP-A No. 2002-3634.
Japanese Patent Application Publication (JP-B) No. 62-34336 and JP-A No. 10-77359 are disclosed foaming and expanding foamed resin beads composed of a core layer including the polyolefin resin and a covering layer prepared from the polyolefin resin having a melting point lower than that of the polyolefin resin in the core layer in order to improve fusion bonding properties between the foamed resin beads.
Although a method described in JP-A No. 2000-290421 prevents the surfactant from transferring (transcription) to packaging products, fusion bonding properties of foamed resin beads are reduced when the hydrophilic polymer is added to the foamed resin beads. Further, since the hydrophilic polymer has water absorption properties, when resin particles are heated in an aqueous medium under pressure, resin particles absorb water. When the expansion ratio is high, the pressure in the cells of the obtained foamed resin beads is reduced by water condensation, which causes the phenomenon of shrinkage of beads in themselves.
In order to achieve a high antistatic performance by the polymer type antistatic agent, it is necessary to add a large amount of the antistatic agent to the foamed resin beads. However, when the additive amount of the polymer type antistatic agent containing the hydrophilic polymer (hereinafter simply referred to as “polymer type antistatic agent” or “antistatic agent”) is increased to obtain a high antistatic performance, the foaming properties of foamed resin beads are inhibited and the fusion bonding properties between the foamed resin beads are reduced with the increase in the additive amount. When the heating temperature at the time of molding is increased to improve the fusion bonding properties between the foamed resin beads, the foamed resin beads cannot resist the heating temperature. As a result, the foamed resin beads are shrunk or fused to not only the surface of them but also the inside of them, thereby causing serious damage to the cell structure of the obtained molded foamed article. On the other hand, when the heating temperature is lowered, fusion bonding properties of foamed resin beads are reduced and the surface condition of the molded foamed article is significantly deteriorated. Thus, a good molded foamed article cannot be obtained. Particularly, the phenomenon is significantly observed in foamed resin beads having a high expansion ratio.
A method described in JP-A No. 2002-3634 involves a process of kneading the polymer type antistatic agent with the adhesive resin and coating the surface of foamed resin beads with the adhesive resin containing the polymer type antistatic agent and thus the production step is complicated. When the adhesive resin is used, foamed resin beads are easily adhered each other in the coating step. Further, the charging property of the foamed resin beads to a molding cavity may be unstable.